Topic 10 - Homeostasis 10.1 – Structure and function of the skin

SUFEATIN SURHAN  BIOLOGY MSPSBS  2010

SYLLABUS CHECKLIST Candidates should be able to: (a) define homeostasis environment;

as

the

maintenance

of

a

constant

internal

(b) explain the concept of control by negative feedback; (c) identify, on a diagram of the skin, hairs, sweat glands, temperature receptors, blood vessels and fatty tissue; (d) describe the maintenance of a constant body temperature in humans in terms of insulation and the role of temperature receptors in the skin, sweating, sweating, shivering, blood vessels near the skin surface and the coordinating role of the brain.

Homeostasis and negative feedback control 

The cells in our body are bathed in fluids (blood, lymph and intercellular fluid). These fluids make up the internal environment. This environment must be kept constant for the healthy functioning of the body cells. Homeostasis is defined as maintaining a fairly constant internal environment (a ‘steady state’)

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Our body undergoes homeostasis by constantly adjusting any changes (stimulus) in the physical and chemical conditions of its body fluids. These conditions include temperature, pH, osmotic pressure and concentration of solutes in the body fluid such as nutrients, urea, carbon dioxide and ions.

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The table below shows several components of the blood which need to be regulated, as well as the organs involved in their regulation: Blood component

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Regulatory Organ

Urea, Water and Salts

□ By the kidneys in the process of osmoregulation

Glucose

□ By the pancreas i.e. via the hormone insulin. □ Insulin converts excess glucose into glycogen, which is stored in the liver and muscles.

Carbon dioxide (HCO3- in the plasma)

□ By the lungs during breathing

Temperature

□ By the skin

The brain is the control centre of homeostasis. The liver, kidneys, skin, lungs and hormones also play important roles in homeostasis.

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Homeostatic processes involve:

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a stimulus (i.e. a change in the internal environment)

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a receptor that detects such stimulus

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a corrective mechanism that is automatically stimulated

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a negative feedback as a result of the corrective mechanism.

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Negative feedback mechanisms are important homeostatic processes. A feedback mechanism is the regulation of a process by the results or products of the process.

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Metabolism is the sum total of all the chemical processes that are continually going on inside living cells. It includes: 

Catabolic processes where complex molecules are broken down to simple ones with release of energy, e.g. cellular respiration and the breakdown of amino acids to urea.

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Anabolic processes where simple molecules are built up into complex ones with an intake of energy, e.g. photosynthesis and synthesis of glycogen from glucose.

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During metabolism, living cells use up water, oxygen and digested food. In the process of using these substances, the living cells produce harmful by-products.

These by-products of metabolism are

considered waste products because they are unwanted by the cells and must be got rid of.

Examples of homeostatic processes 

Carbon dioxide level: Changes in the carbon dioxide concentration of blood automatically affect the rate of breathing. When exercise causes an increase in the concentration of carbon dioxide in the blood, the rate of breathing increases. This decreases the carbon dioxide concentration in the blood back to normal. The restoring of the normal state gives a negative feedback which switches off the mechanism that causes the rate of breathing to increase.

Regulation of blood glucose level: The normal level of glucose in human blood maintains around 70–90mg/100cm3. Any changes in the blood concentration of glucose is detected by the pancreas which immediately secretes hormones that bring out the corrective mechanism as shown below: 1. An increase in blood glucose level (e.g. after eating) results in insulin secretion which causes the conversion of excess glucose to glycogen. 2. A decrease in blood glucose level (e.g. during starvation) will cause the pancreas to release glucagon. Here, stored glycogen in liver is converted to glucose which is then released into the blood stream In both cases, the level of glucose in the blood stream returns to normal (negative feedback). 3

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Regulation of water potential: The urine composition is adjusted to ensure that the water potential (osmotic pressure) of blood remains constant. 1. The water potential of the blood depends on the amount of water and salts in the plasma. 2. The amount of water in the blood is controlled by antidiuretic hormone (ADH). 3. ADH is produced by the pituitary gland in the region of the brain called the hypothalamus. 4. ADH increases water reabsorption by the kidney tubules.

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Body temperature control: This is kept at about 37oC. The skin plays an important role in this homeostatic control. The importance of maintaining a constant body temperature: 1. To maintain the optimum temperature for metabolic enzymes to work in. 2. Allow an organism to be active throughout the year, regardless of changes in the environmental temperatures. 3. Allow an organism to feed throughout the year. 4. Enable the organism to move and colonise new areas in order to find new food supply and habitat for better survival.

Structure of the skin

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The skin is the largest organ of the human body

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It forms a physical barrier between the body and the external environment

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It is an important sense organ

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It is particularly important in the homeostatic process of temperature regulation

Changes in the blood temperature are detected by the hypothalamus situated on the underside of the brain. This is the structure responsible for the coordination of all temperature control mechanisms.

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(A) EPIDERMIS 1. Cornified layer 

Made up of dead, flat, hard, dry and horny cells

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The cells are hard due to the deposition of a protein called keratin (also found in hairs and fingernails)

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Functions: a. To provide a water-resistant / waterproof layer to the skin so as to prevent excessive water loss from the skin by evaporation b. To provide a physical barrier in preventing the entry of harmful microorganisms into the body as well to prevent mechanical injury to the inner skin layer. -

The cells in this layer are continuously rubbed off but are replaced by new cells from beneath this layer

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The cornified layer is the thickest at the regions of the body where there is high pressure and wear and tear such as at the soles of feet and the palm.

2. Granular layer 

Made up of living cells

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The cells move upwards towards the cornified layer, where they become progressively harder with increased deposition of keratin and also become further away from the blood supply, hence, resulting in their eventual death.

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Function: To continuously replace dead cells removed from the cornified layer

3. Malpighian layer 

Made up of living cells

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Function: These cells constantly divide in order to provide new cells to the top layers (granular and cornified layers)

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The cells in this layer have coloured pigments called melanin

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These pigments provide colour to the skin

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Function:

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To protect the DNA in our body cells from damage by ultraviolet rays / light

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(B) DERMIS (Inner or true skin) 

Made up of fibrous connective tissues, collagen fibres, blood vessels, hairs, sebaceous glands, sweat glands and nerve endings.

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Upper parts are arranged in ridges called papillae. These are responsible for the pattern of the fingerprint.

1. Blood vessels 

Dermis is supplied with arterioles which branch into capillaries as they pass through this layer.

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The arterioles are involved in temperature regulation as they can change the diameter of their lumen, thus the amount of blood (= amount of heat) flowing to the skin. This change is either vasodilation (increase in diameter of lumen) or vasoconstriction (decrease in diameter of lumen).

2. Hairs 

Malpighian layer of the epidermis sinks into the dermis to form a hollow tube which has a structure called hair follicle at the base.

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At the base of the hair follicle is a mass of blood capillaries and nerves.

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Attached to the hair follicle is the hair erector muscle. When this muscle contracts, it becomes short and thick, pulling the lower part of the hair. This causes the hair to stand on its end and goose pimples can be seen.

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The standing hairs are useful in trapping a layer of air on the surface of the skin to act as a heat insulator (not effective in temperature regulation in human).

3. Sebaceous glands 

They secrete an oily substance called sebum into the hair follicle, which eventually spreads onto the surface of the skin and hair.

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Functions: a. To lubricate skin and hair making them smooth and soft. b. To provide a waterproof layer to the skin in order prevent dehydration of the skin from excessive water loss by evaporation. c. To prevent bacterial growth (has antiseptic property).

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4. Sweat glands 

The sweat glands are closely associated with blood capillaries.

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Some water, urea and salts diffuse out of the blood capillaries into the sweat glands forming sweat (Excretion).

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Sweat move out of the glands through the sweat ducts and out onto the surface of the skin through the sweat pores.

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Sweat is important in cooling the body / prevents the body from overheating.

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Heat energy from the body evaporates the water molecules in sweat into vapour, which then move away from the body taking away with them the heat energy (important in temperature regulation in human).

5. Nerve endings 

These are ending of the nerve cells / neurons.

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They become receptors or sensory nerve endings to detect various parameters such as pain, changes in temperature and pressure.

Functions of the Human Skin 1. Acts as a barrier between the body and the environment. It protects the body from invasion of bacteria and harmful substances. The epidermis bears scratching, cuts, bruises, burns and other injuries. 2. Acts as an organ of excretion. It excretes water, mineral salts and urea through the sweat pores in the form of sweat. 3. Acts as an organ regulating body temperature. Sweat glands cause the skin to lose heat by the evaporation of sweat. The hair and fat cells help to keep the skin warm. 4. Acts as a sense organ. Sensory nerves in the skin receive sensations that are transmitted to the central nervous system. The motor nerves respond to stimuli. 5. Acts as an organ for storage for water, salt and fat. 6. Manufactures Vitamin D when skin is exposed to sunlight.

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Maintenance of a constant body temperature in man On a cold day

On a hot day

“Overcooling – the body loses heat faster than it can produce”

“Overheating – the body produces more heat than it is losing it e.g. during vigorous exercise”

(a) Detection by skin receptors and hypothalamus in the brain

Messages from the temperature receptors of the nerve endings of the dermis of the skin and the hypothalamus are immediately sent out in various nerves to switch on the „warming mechanism‟.

Messages from the temperature receptors of the nerve endings of the dermis of the skin and the hypothalamus are immediately sent out in various nerves to switch on the „cooling mechanism‟.

(b) Blood vessels (arterioles and capillaries)

Vasoconstriction – muscles in the walls of the superficial arterioles and capillaries of the skin become narrower so that less blood flows through the capillaries of the skin, and less heat is lost by convection and radiation.

Vasodilation – muscles in the walls of the superficial arterioles and capillaries of the skin become larger or dilate so that more blood flows through the capillaries of the skin, and more heat is lost by convection and radiation.

(c) Hairs

Erector muscles contract causing hairs to stand on end. The layer of air trapped between the hairs becomes thicker. Since air is a poor conductor of heat, it insulates the body. This response is most effective in furbearing animals.

Hairs lie flat because the erector muscles relax. The layer of air trapped is very thin. Thus heat is easily lost from the body by radiation and convection.

(d) Sweat glands

Sweat production stops so that the latent heat is not lost from the body or heat lost by evaporation of sweat is reduced.

The sweat glands are activated producing more sweat glands are activated producing more sweat onto the skin surface during vasodilation. When sweat evaporates from the skin surface, it takes latent heat away from the body and so cools the skin and body down.

(e) Metabolic rate

Fat respiration increases. Respiration speeds up the fat and liver cells. They are broken down to produce glucose to release more heat. The heat is distributed all over the body via the bloodstream to warm the body.

Metabolic rate decreases and less heat are produced to prevent the body from becoming overheated.

(f) Muscles

Shivering due to involuntary contraction of the muscles occurs. Uncontrollable bursts of rapid muscular contractions and relaxations of the limb muscles release heat as a result of chemical changes and help to raise the body temperature to normal.

Structures involved

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HOMEOSTASIS – REGULATION OF BODY TEMPERATURE

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